1. Field of the Invention
The present invention relates to a network system and a network apparatus, and particularly to a technology for improving the redundancy and the forwarding capability by enabling plural network apparatuses to operate virtually as one network apparatus. More particularly, the invention relates to a technology for maintaining the redundancy and preventing the lower forwarding capability even when a fault occurs on a private line connecting plural network apparatuses through which the control signal is exchanged.
2. Description of the Related Art
In recent years, the number of links to accommodate and the forwarding capability required for the network apparatus such as a router switch have been improved. Therefore, there is a demand that the number of links to accommodate and the forwarding capability can be easily extended without greatly changing the existent network configuration as the scale of an office network increases, for example. In this background, a network system to extend the number of links to accommodate and the forwarding capability by enabling plural network apparatuses to operate as one virtual network apparatus has been proposed.
For example, in a network system called a Virtual Switching System (VSS) as described in non-patent document 1, two chassis-type layer 3 switches (hereinafter referred to as a physical switch) can be operated as one virtual switch (hereinafter a virtual switch).
In the VSS, a control plane (for performing the apparatus control and the protocol process) of two physical switches is divided into active and standby, and a data plane (for performing the packet forwarding) for either is used in the active state, as shown in FIG. 2 of non-patent document 1. Also, in the VSS, it is recommended as a method for connecting to an adjacent server or network apparatus that an adjacent apparatus and each physical switch of the VSS are connected via an individual physical line, and the line is laid by a link aggregation (LA), the LA spanning the physical switch of the VSS. By adopting this connection method, traffic is distributed over two physical switches in transmitting the packets owing to a load balance function of the LA, whereby the forwarding capability for two switches can be effectively utilized. Further, using a unit spanning LA in which each physical switch and the adjacent apparatus are connected via the physical line, and the physical lines are configured as the LA, the VSS and the adjacent apparatus are connected virtually via one line, resulting in a loop-less and redundancy configuration, whereby it is unlikely that the network flutters at the time of fault, as seen in the other L2 redundancy protocols.
It was described that in the VSS, to realize the virtual switch, two physical switches are connected via a private line called a Virtual Switch Link (VSL), as shown in FIG. 4 of non-patent document 1, to exchange the VSS control traffic and the ordinary data traffic (refer to page 7 of non-patent document 1, “Virtual Switch Link”). Also, it was described that the VSS control traffic includes a packet of the VSL control protocol called a Virtual Switch Link Protocol (VSLP) and a control signal exchanged within the apparatus (refer to page 51 of non-patent document 1, “Control Traffic over VSL”). In the following explanation, the VSS control traffic and the ordinary data traffic via the VSL are generically called the VSL traffic.
When the VSL fails but two physical switches do not fail, each physical switch cannot exchange the VSL traffic as described above. Thereby, each physical switch falsely detects that the other physical switch fails, and the standby physical switch switches the self-unit to active, resulting in a state where two active physical switches exist on the network. Thereby, two network apparatuses having the same address exist, and various problems such as a packet loss and false forwarding may possibly occur. To avoid those problems, the VSS has a function of detecting that each physical switch does not fail and only the VSL fails. If a VSL fault is detected using this function, the physical switch in the active control plane shuts down all the ports of the self-unit, and the physical switch in the standby control plane transits to active, thereby avoiding a state where two active apparatuses exist. In the VSS, the state where all the ports are shut down due to occurrence of the VSL fault is called a recovery mode (refer to page 42 of non-patent document 1, “Detection Mechanisms and Configuration”).